Synchronized radio transmission



F 10, 1948- w. 1.. BARROW I 2,435,569

SYNCHRONI ZED RADIO TRANSMISS ION Filed Aug. 5, 1941 3 Sheets-Sheet l2 I F I Er. i

v II I 13 TRANSMITTER f o JCSCILLATOR 7 0 I I I mu tm (f +f, or (f -f,

MODULATOR FILTER CONVERTER I I I \30 I I I MODULATION 1Q LEAMPLIFIER OscILLATOR AND I 7 I M DEMODULATOR I I I I PULSE '17 INDICATOR GENERATOR LPULsE I I I I -tPU l3 PULsE MODULATOR 22 I I iPULS Iv TRANSMITTER oscILuATOR cONvERTER /8 I I I o (fowl) f =L ULsE| I I MODULATION or V I.EAMPLIFIER MODULAT R i osclLLAToR I [6/ AND f1 I 30 v DEMODULATOR D In; n ki' I LI FILTER INDIcAT R F I E- E.

INVENTOR.

WILMER L. BARROW .HIS ATTORNEY.

Feb. 10, 1948. w. L. BARROW 2,435,569

SYNGHRONIZED RADIO TRANSMISSION Filed Aug. 5, 1941 s Sheets-Sheet 2 TRANsMITTE I OSCILLATOR MODULATOR I I f O I 1 4 gggtitglg'; i BALANCED OSCILLATOR f1 MODULAT R f2 ci i a CONVERTER, PHASE I.I-'.AMPLIFI R, DIFFERENCE DEMODULATOR f INDICATOR 25 Z r I I o'fio i.) MODULATION MULTIPLIER OSCILLATOR MODULATOR X" f, I R

27 I [9 f nf nfi if 2a TRANSMITTER OSCILLATOR r10 18 f I o MODULATION BALANCED 5 MODULATOR I l v r in FILTER CONVERT R. MULTIPLI R LEAMPLIEIER, INDICATOR 'Xn I EMODULAT R f s 2 INVENTOR. 27 n(f I 25 wILMER L- BARROW HIS ATTORNE Feb. 10, 1948. w. 1.. BARROW 2,435,569

SYNGHRONIZED RADIO TRANSMISSION Filed Aug. 5, 1941 5 Sheets-Sheet 5 a F l El' E1 TRANSMITTER OSCILLATOR m l r MODULATIQN PHASE AND OSCILLATOR MIXER AMPLITUDE =5 ADJUSTER (f if l CONVERTER,

FILTER l.F.AMPLlF |ER.

DEMODULATOR INDICATOR 7 INVENTOR. WILMER L BARRow H IS ATTORNEY, I

Patented Feb. 10.1948- "-i SYNCHRONIZED RADIO TRANSMISSION Wilmer L. Barrow, Newton, Mass assignor to -Sperry Gyroscope Company. Inc., Brooklyn,

N. Y., a corporation of New York Application August 5, 1941, Serial him-105,466

This invention relate mama transmissionsystems: and it refers n iore particularly to cir-' cuit arrangements for synchronizing the frequencies of currents at the transmitting and receiving ends of a transmission circuit. As a distinctive ternrnl have applied the name syn chrodynefto the class of circuits disclosed here-:

in for accomplishing this result. H

Synchrodyne? circuits obstacle detection and distance measuring systems in which microwaves; are reflected to the receiver from a remote object, for example, an

aerial target. In, the past, mic ro-wave receivers used in connection withsuch systems have been of the types commonlyknown as straight detector, tuned radio frequency amplifier and superheterodyne receivers, Difficulty has been ex perienced in securing stable reception with receivers of anypf these types since the trans- V are particularly adapted to provide stablereception for microwave signals and -find great usefulness in.

r 16 Claims. (Cl. 2501.64)

mitting frequency in general is not absolutely steady nor is the transmitted wave' purely. sinusoidal in character. At ultra-high frequen-' cies the transmitter frequency'issubject to vari ation from a number of causes suchas change in supply voltage, thermal drift, vibration and other effects. To receive signals from a transmitter whose frequency is subject to drift, it has previously been necessary to employ areceiver having a band width much greater thanthe nominal frequency band of 'the received signals, in order to allow for drift. A disadvantage of a wide band receiver lies in the resulting unfavorable signal to noiseratio, since this ratio is generallylower as the band width becomes greater, and in the greater number of stages of vacuum tube amplification required, since the gain per stage decreases as the band width in creases. When a superheterodyne receiver is used, even though the transmitter frequency is held constant, great difficulty is experienced in s'ecurings'ufiiclentstability of the "receiver local oscillator to make practical the use of a narrow band intermediate frequency amplifier. For

micro-wave reception, this imposes severe conditions on the design and operation of a receiver of this type. a a

One object of the present invention is to provide a receiving system which permits theuse of a very narrow receiving band width, i. e., a high degree of selectivity at thereceiver, without at the same time imposing undue frequency stability restrictions on either the transmitter or Another object is to obtain an improvement in the signal to noise ratio of a superheterodyne' type receiver-"at ultra-high frequencies by 'per-.

mittin'g'a narrowband width intermediate freduency amplifier to'be used.

Another object is to provide an'ob'stacle detec-f tion system making use of reflected radio wavesof ultra-high frequen'cyin which'the tuning of r the detection system is constantly and automat-' ll) ically maintained.

A further object is to supply-a stabilizing ire-6 quency to the receiver of a radio transmission system over a path other than the normal transmission path for securing a favorable signal to noise ratio by permitting increased selectivity of;

the receiver.

In the drawings, a r V Fig. 1 represents schematically a system for practising my invention, which I term the basic synchrodyne' circuit, wherein a radio transi mitter is connected to a receiver by 'a path supplemental to the radio transmission path, which may change direction due to reflection, for sup-' plying a stabilizing frequency to the receiver. 7

Fg. 2 illustrates schematically a modification of my basic circuit wherein the transmitter car rier is modulated by a pulse or series of repeated} pulses.

Fig. 3 illustrates schematically a distance measuring system utilizing the principle of my invention, in which the phase difference of the transmitted and received waves is utilized to indicate distance to a reflecting objector surface."

Fig. 4 is a schematic representation of another modification, wherein modulation is accomplished at alow frequency and frequency mul- ,tipli'cation is employed to secure a high transmitting frequency. 7

Fig.5 represents schematically a further modification wherein a high "frequency bridge circuit composed of transmission lines is utilized as a balanced or carrier suppression modulator.

Referring tofFig. 1, there is illustrated a trans mitting oscillator 10' for generating a carrier frequency is, which may be infthe ultra high fre quency or micro-wave range, as is commonly the case in reflection systems for obstacle or target detection or for distance measurement. Anadjustable directional radiator ll of any suitable type, as for example that described by L. J. Chu and W. L. Barrow in a paper entitled Electromagnetic horn design, publishedin Electrical Engineering, vol. 58, pp. 333 to 338,, is connected to oscillator l0. V

The wave radiated from antenna or horn 'l I at frequency In, is illustrated as being directed toward an obstacle or target such as airplane l2 and the radiation reflected therefrom as being received by a suitably oriented directional receiving antenna or horn l3 which supplies a signal to a receiver l4 operating on the superheterodyne principle, of which the high frequency converter or first demodulator (hereinafter referred to as the converter) and the intermediate frequency amplifier and second demodulator are indicated by blocks l5 and I6 respectively. It is to be understood that the intermediate frequency amplifier includes an intermediate frequency'filter limiting the frequency band. passed, as is com-. mon in receivers of this type. The output of the second demodulator (hereinafter referred to as "the demodulator), is received by an indicator I! which may be a meter or recorder of suitable type for exhibiting the receiver output.

At the transmitter, a source of modulation frequency such as modulation oscillator l8 supplies a modulation frequency .fi to-moduator [9 which 7 also" receives: frequency in from transmitter osciliator' ii]. The output of modulator l9 includes a component at the frequency fo'and in addition components representing the sum-and difference of f0 andji which will be referred to as the upper and: lower side frequencies, respectively. Frequency i1 :is normally too low to appear as a component of appreciable magnitude in the output ofa; modulator designed for high frequency operation, When the arrier is modulated by a band: of frequencies instead of by a single frequency then in place of side frequencies there appear upper and lower side bands, as is Well known in the art. The output of modulator I9 is received by band pass filter 20 which selects one or both .of the side frequencies and the output of the filter is transmitted by a, wire line-or supplemental radio path to frequency converter 5 for heterodyning therein the reflected :fnwave received by horn 13 in the manner that the out- Put of the local oscillator is utilized in the usual type of superheterodyne receiver.

In operation, assuming anunmodulated carrier f0 to be employed; and if the transmitting antenna; or other radiator H is suitably oriented, for example, as the result of searching until a reflecting object is located, the radiated wave will strike obstacle l2 and a portion of the energy will be reflected to receiving antenna [3 and a signal will be applied at frequency in to frequency converter l5. This converter also receives one. of the side frequencies, for example, the upper side frequency .fo+.f1, and as the converter is a modulator, that is a. non-linear device, the difference of the carrier and sidev frequency will appear as one component in its output, this difference being the modulating frequency f1. It will be apparent that in the arrangement described the frequency stability of the signal passed to the intermediate frequency amplifier is independent of the frequency stability of the transmitting oscillator and depends only on the stability of the modulation oscillator. Since the latter oscillator operates at a relatively low frequency, its stability can be made high. By thus supplying a stable intermediate frequency signal, anarrow band filter can be used in'the intermediate frequency amplifier with. a resulting favorable signal to noise ratio, and also a relatively small number of stages in this amplifier. I

While the heterodyning side frequency is sup-- plied to converter [5 at all times, due to thedirectional characteristics of the transmitting and receiving antenna no fo carrier is received unless the direction of transmission is altered by reflection and therefore an indication on meter I1 is obtained nly when a reflecting obstacle is in the radiated field. If a variable amplitude signal is radiated by the transmitter, a received signal varying proportionally in amplitude but independent. of frequency distortion due to drifting of the transmitter frequencymay be obtained by transmitting a constant amplitude heterodynlng side frequency to the frequency converter of the receiver.

Transmitting and receiving antennae II and 13 are shown vin'Fig. 1 as spaced from one another and directed substantially in opposite directions. In many arrangements, however, particularly when horn type. radiators and receivers are employed the radiators may be separated by a relatively small distance and hence directed at substantially the same angle, particularly when transmitting to and receiving reflected radiation from an objectat a considerabledistance.

Referring .now more particularly tothe arrangement of Fig, 2,. this is in general similar to that of Fig. .1 except that the signal supplied by transmitting. oscillator in to transmitting antenna ll is a modulatedsignal, the modulation here being shown as derived from. pulse generator 2| connected to pulse modulator 22. The pulse method is peculiarly adapted for measuring distance to a reflecting object or surface and is well known in the art. It .has been widely employed in one. form to measure the height of the ionosphere and variousv modifications .of this method have been proposed, or used, to measure the distance between an airplane and the earth, the transmtter being located in some cases on the planeand in some cases on the earth. Such systems of measurement require a high degree of frequency stability, which the "synchrodyne circuit is well adapted to supply.

The operation of the system of Fig. 2 will be apparent from the described operation of that of Fig. 1. A series of recurrent pulses, which may each have a duration of the order of a few microseconds, is applied by generator 21 to pulse modulator 22 to produce corresponding pulses of the current at the frequency supplied by transmitting oscillator Ill and the. modulated wave after reflection from reflecting surface l2, supplies the transmitted frequency .10 to frequency converter l5 which also receives one or both side frequencies resulting from the modulation of in by a sinusoidal frequency ii in modulator l9. Converter l5 passes to the intermediate frequency amplifier a pulse-modulated signal, whichv upon demodulation reproduces the original series of pulses as indications of indicator l1. As in the system of Fig. 1, the high degree of frequency stability secured permits a high signal to noise ratio to be attained by the use of a narrow band intermediate frequency filter. which is of great importance in pulse measurement systems, and the indication of the puses is not distorted by frequency drift.

In Fig. 3 there is illustrated the application of the synchrodyne. circuit to a system for measuring distance to a reflecting object by indicating the difference in phase of a transmitted and reflected wave, Distance to the objectis proportional to the phase shift of the modulation envelope of the received wave relative to that of the transmitted wave, and this method has been suggested for the operation of radio altimeters and similar apparatus. In measuring arrangements of this type, stabilizing arrangements are 0.! great utility to; eliminate spurious: frequency drift. this: providing. greater sensitivity andf i'ncreasedrange. v

Transmitting antenna H and? receivingan' tennai [3 are suitably oriented to radiate a; wave toward reflecting: object 23: and. to receive the portion" reflected therefrom. Carrier frequency forsuppli'ed by transmitting oscillator l0,xaccord 'mg;torithearrangementryof Eig- 3',;is modulated.

in; modulator. l9 at: a lower: frequency 12: supplied from antoscillatoror othersouicefl; Current due: to the modulated. wave? reflected: from object. 23. and recelvedrbyzantenna: t3- is" supplied; asin previously described arrangements; to a super heterodyneztyne receiverfthe converter; inter mediate frequency amplifier; and demodulator of which are'represented by block' 25.. The heterodyning frequency for the converter is obtained, iirprinciple; asher'etofore'described. In the: arrangement ofv Fig.1'3. the modulation. of jcby f1, however; isz shown as accomplished in: balanced modulator 2th this'being aitypeof modulator well known. in theart. which; bysuitable connections, suppresses thecarrier frequency and passes only thegsidefrequencies; or side-.lban d's, thus doing away with the: use of filter 20' of Figs. 1 and- 2-; Obviously, theicombinatiorrof' unbalanced modulator and filter, previously described, may alternatively be: used ill'the arrangement; of- Fig. 3 or a balanced modulator may be employed for suppression: of the carrier frequency [him any other system: described herein.

The output of thea receiver demodulator includes: a component atthe frequency i2, and other. component's:.may* be eliminatedby' welllmown means; The demodulated frequency fa is. applied to: phase; difference indicator 2T, which also receives asignaliat frequency J: from oscillator -24; Phase-difference indicators are known in the. art as'. for example, the type described b J..R;-G.' JPBIancardeta-I; hr U. S. Patent-No. 1,968,068,1-ssuedduly 31, I934. If f2 is chosen as the frequency-of commercial power circuits, other wellcknownu instruments are available for indicating phase difference; It will' be apparent that thephase diiferencc indicated-as that due tothe distance traveled by theradiatedwave before and after reflection. froin the reflecting object, and thisditference, therefore, furnishes am'ea'sure of the distance of the object from the measuring apparatus. v

Technical difliculties ii -the construction and operation of a carrier suppression modulator, or its equivalent, at ultra high radio-frequencies, make it desirable in some cases to carry out the various modulating processes employed in connectionwiththe arrangements described herein at frequencies lower than the radiated frequency. An arrangement for accomplishing this is shown in Fig, 4 in connection with an obstacle detection system in which transmitting; oscillator operates at a relatively low frequency in comparison withthe radiatedi'requency which oscillator, frequencyiis.inodulated' inv modulator: is by the still'lower frequency; a supplied by modulation oscillator 21. The frequency of the resultin modulatedwave is then increased'by frequency multiplier, 28; Frequency multipliers are well known in the art, a high frequencies being described in U. S. Patent 2,086,904, issuedto J.. Evans, July 13, 1937. i A similar process of. frequency multiplication is employed in obtaining: the side. frequency for heterodyning I purposesin. receiver annul: one; another.

inultiplicotion being accomplished: by multiplier tion in which modulation-is accomplishedby the use of ahigh frequency transmission line bridge circuitwhich includes a non-linear element at one of the bridge points. Bridge circuits employ-ing transmission lines are described inmy copending application Serial No. 376,253,, filed January-28 1941, Patent No. 2,416,790 granted. March, 4,1947, and as. there: disclosed comprise four transmission lines. having distributed electric constants, such as lines 30,, 3| 32, 33' schematically represented in Fig. of the present-apzilication, connected in tandem to form a. closed bridge: circuit. The construction of these lines. ma vary widely to include ordinary two wire lines, shielded conductor or coaxial lines,- hol.- low pipe lines, dielectric guides, and: other meansfor guiding electromagnetic 'waves known to those skilled in the art. p

The four lines ofFig. 5 form two parallel transmission paths between transmitting oscillator. and. a loadv (here illustrated as filter 34 in tandem with: receiver comprising lines 30, 3| and1ines. 32,- 33,- respectively.. If these twotransmission paths have similar characteristics an E. M. F. impressed across the junction of lines 30' and 32 will initiate the transmission of electromagnetic waves over the two paths which upon arrival at the=fi1ter'-34 will have equal amplitudes and by suitable; connections may be; made to oppose and Phase and amplitude adjuster provides the: means for adjusting the characteristics of the transmission path, 32, 33

to: match: thatof the parallel path 30,31. When suitable adjustments are made so that the two waves cancel one another direct transmission of energy from oscillator ID to filter 34 at the frequency of the oscillator is prevented. Mixer circuit 35 at the junction of lines 30 and 3| is a non-lineardevice of any suitable type: which receives' an E. M. F. at frequency h'from modulation oscillator l8 for combination with the E. M. F. at frequency fodue to the arrival at the mixer of the wave travelling over line 30. The result of: modulation in mixer 35 is a series of modulation products from which filter 34 may select,

for example, the. upper side frequency (fn+f1) which. is then applied to the frequency converter of receiver circuit 25 for" heterodyning the frequency fcreceived. by antenna l3 as in previously described circuits. By the use of transmission lines it is feasible to construct abridge circuit which operates asa carrier suppression type modulator at. extremely high frequencies and which retains its balance over a frequency range of considerable width.

many changes could be made in the above construction and many apparently widely different-embodiments of this invention could be made without departing from the scope thereof, it is ihtended'th'a't all matter contained in the above typesuitable for operation at description or shown in the'accompanying drawings'shall be interpreted as illustrative and not in ali-niiting sense.

Having described my invention, what I claim and desire to secure by Letters Patent is:

1. A systeinfo'r measuring'distance to a refleeting surface comprising a radio transmitter including a source of high frequency, means for 2 thi's frequency 7b generating recurrent, pulses of short duration, a

aces-sea modulator connected for modulating 'saidhigh' frequency by said pulses, a directional radiator connected to receive the output of said pulse modulator for selectively radiating a pulse modulated wave in the direction of the reflecting surface, a directional receiver receiving radiation reflected from said surface, said receiver being of the superheterodyne type, circuit means for supplying a heterodyning frequency derived from the transmitter frequency to the converter of said receiver, including a source of low frequency modulation, a modulator receiving alternating potentials at said high frequency and said modulating frequency; filter means for selecting from the output of said modulator at least one of the resultingside bands, circuit means for applying said side band to said frequency converter jointly with'the received radio signal, said receiver in-- cluding a demodulator for demodulating said converter output, and means for indicating said demodulator output.

1 2. A system for determining the position of an objectcapable of reflecting radio waves comprising a radio transmitter including an oscillator, a source of low frequency alternating potential. a

modulator connected to said oscillator'and sourcev and a directional radiator for selectively radiating in the direction of the object, a directional superheterodyne receiver receiving radiation reflected from the object including receiving antenna means and a frequency converter connected thereto, circuit means for supplying'to said converter the received radiofrequency and a heterodyning frequency suitable to produce the intermediate frequency. of said superheterodyne receiver including a modulationsource. a modulator connected to said oscillator andto said mod-- ulation source and means for suppressing'the oscillator frequency in the output of said modulator, and a phase diflerence indicator jointly actuated by a potential derived from the output of said converter and a potential from said'low fre-' quency source for indicating the phase difference of the common frequency of said two potentials.

3. An obstacle detection system comprising a transmitting oscillator, a modulation oscillator. a modulator connected to said two oscillators, a

frequency multiplier connected to'the output of said modulator, an adjustable directional an-' tenna receiving the output of said multiplier for selective radiation in the direction of the obsta-' cle, a second modulation oscillator, a balanced modulator connected to said oscillatorand saidsecond modulation oscillator, circuit means' for' selecting one of the side bands from the output of said modulator, a second frequency multiplier for multiplying the frequencies of said side bands, a

directional receiving antenna receiving. radiation reflected from the obstacle including a frequencyconverter connected to said second frequency multiplier and said receiving antenna, and-means for exhibiting the output of said converter.

- 4. A radio transmission system comprising a transmitter having a transmitting oscillator, a-

frequency multiplier for increasing the frequency thereof and an antenna receiving the output of said multiplier, a receiver including an antenna receiving signals from said first antenna and a frequency converter-,a source of modulation having a frequency lower than that of saidloscillator, a modulator connected to said oscillator and said source, circuit means for suppressing the fre-.

quency of said oscillator in the output of said modulator, a second frequency multiplier for increasing the frequency of said modulator output,"

circuitmeans for supplying the output of said multiplier to said frequency converter for altering the frequency of the received radio signals, and" means for exhibiting the output of said converter.

5. An obstacle detection system including a transmitting oscillator, a directional antenna connected thereto for selectively radiating in the direction of the obstacle, a superheterodyne receiver receiving radiation reflected from the ob-' stacle including a receiving directional antenna and a frequency converter connected thereto, carrier frequency suppression circuit means conmeeting said oscillator and said converterincluding a balanced bridgecircuit, a source of m0du-,

lation frequency equal to the intermediate fremodulation thereof to.said; converter.

6. Anobstacle detection system as claimed in claim 3-in which. said bridge circuit comprises:

transmission linesas arms thereof, the characteristics of said, transmission lines being chosen to convey said transmitter. oscillator frequency, substantiallywithout loss. i

7. An obstacle detection system as claimed in claim 3 in whichsaid-bridge circuit comprises transmission lines as arms thereof "and in which phase and amplitude adjustments are provided for-balancing said bridge circuit;

-:-8. Ina radio transmission system comprising ahigh-frequency source of nominally constant frequency but subject to frequency drift and means supplied from said source for radiating a wave at the frequency thereof, a superheterodyne receiverof energy radiated by said means including a frequency converter, a source ofmodulation frequency harmonically related to-the intermediate frequency of said receiver, a modulator, circuit means for supplying as inputs to said modulator a potential derived from saidmodulation source and a potential derived from non-radiated energy supplied by said high frequency source, circuit means for applying as inputs to said converter the received wave and a sideband component of: the output of said modulatorto obtain as an output therefrom a signal insensitive to frequency drift of said high frequency source, and;con-- nected means-actuated by said signal.

. 9. In a radio transmission system comprising ahigh frequency source of nominally constant frequency but subject to frequency drift, means connected to said source for radiating a wave at the frequency thereof and means for producing an amplitude modulation of the radiated wave, a

superheterodyne receiver of energy radiated by said radiating means including a frequency converter, a source of modulation frequency equal to. the. intermediate. frequency, of. said receiver, a] modulator,'circuit means for supplying as inputs. to said modulator a potential. derived fromsaidmodulation source and a potential derived from non-radiated energy supplied by said high frequency source, circuit means. for applying as inputs to .said converter the received wave and a sideband component of the output of said modu-. lator to obtain as an output therefrom a signal reproducing amplitude variations of the radiated wave butinsensitive to frequency drift of said high frequencysource, and connected means actuated by said variable amplitude signal.

'10. ms; system'for transmittinghigh" fre quency energy to an obstacle and receiving a reflected portion thereof, a transmitter including a high frequency oscillator and a radiator supplied therefrom at a nominally constant carrier frequency subject to random drift, a superheterodyne receiver including a frequency converter, a source of modulation frequency equal to the intermediate frequency of said receiver, a modulator, circuit means for supplying as inputs to said modulator a potential derived from said modulation source and a potential derived from non radiated energy supplied by said oscillator, means for selecting from the output of said modulator a sideband component, and circuit means for applying said selected component to said converter for reproducing said modulation frequency by combination with the received wave.

11. The method of obtaining a variable amplitude, constant first frequency signal corresponding to a variable amplitude signal radiated by a transmitter of varying carrier frequency comprising the steps of receiving the varying carrier frequency signal, modulating the varying transmitter carrier frequency at a substantially constant frequency intermediate to said first frequency and said carrier frequency, sending from the transmitter to the receiver a non-radiated signal of constant amplitude at one of the varying side frequencies resulting from said modulation and combining said received varying frequency signal with said varying side band signal to obtain said constant intermediate frequency signal varying in amplitude in correspondence with the radiated signal.

12. The method of receiving a substantially constant first frequency signal from a remote radio transmitter of slowly drifting carrier frequency, said signal having an amplitude substantially independent of said drift, comprising the steps of receiving a signal at the drifting transmitter carrier frequency over an unguided path, modulating the varying frequency at the transmitter at a substantially constant frequency intermediate to said carrier frequency and said first frequency, transmitting signals at one of the resulting side frequencies to the receiver over a guided transmission path and there combining the signals received by said two paths to obtain said constant intermediate frequency signal having an amplitude proportional to the amplitude of the radiated signal, and demodulating said intermediate frequency to obtain said first signal.

13. An obstacle detection system comprising a transmitting radio frequency oscillator, a directional radiator supplied thereby at substantially a single carrier frequency for selectively radiating a wave at said carrier frequency in the direction of the obstacle, a directional receiver of the superheterodyne type receiving radiation reflected from the obstacle and including a frequency converter, a source of modulating frequency equal to the intermediate frequency of said superheterodyne receiver, a modulator connected to receive potentials of substantially single frequency derived from said oscillator and said modulating source, circuit means for supplying said converter with a potential at one of the side frequencies resulting from modulation in said modulator jointly with a potential derived from the received radiation, and means for exhibiting the magnitude of the component in the output of said converter having a frequency equal to the difference between that of said radio signal and said side frequency.

14. An obstacle detecting system comprising a radio transmitter having a high frequency source and a directional radiator connected thereto for selectively radiating a wave of nominally constant carrier frequency in the direction of the obstacle, a directional superheterodyne receiver receiving radiation reflected from the obstacle including a frequency converter, a source of frequency equal to the intermediate frequency of said superheterodyne receiver, a modulator connected to receive as joint inputs potentials at the frequencies of said high frequency and intermediate frequency sources, respectively, a band.- pass filter for selecting from the output of said modulator one of the side frequencies, and circuit means for supplying the output of said filter to said converter jointly with the received signal at the frequency of said transmitter for reproducing the said intermediate frequency.

15. A target locating system comprising a transmitter having a transmitting oscillator and a directional radiator connected thereto for radiating a carrier wave of nominally constant frequency but subject to frequency drift, said radiator being adjustable for selectively radiating in the direction of the target, a directional re-' ceiver for receiving radiation reflected from the target, said receiver being of the superheterodyne type and including a frequency converter for reducing the received signal to an intermediate frequency, means for supplying a heterodyning frequency to said converter including a source of modulating frequency harmonically related to said intermediate frequency, a modulator receiving as inputs a substantially constant carrier frequency potential from said transmitting oscillator and a potential derived from said source of modulation, filter means for selecting one of the side frequency from the output of said modulator and circuit means connecting said filter and converter, a relatively narrow band intermediate frequency filter, means for demodulating the signal passed by said filter, and indicating means responsive'to the output of said demodulator.

16. A position detecting system as claimed in claim 15 in which said modulator is a balanced modulator connected for suppressing the frequency of said oscillator.

wmmm. L. BARROW.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,024,614 Terry Dec. 17, 1935 2,236,893 Chafiee Apr. 1, 1941 2,231,929 Lyman Feb. 18, 1941 2,183,399 Heising Dec. 12, 1939 2,271,519 Wolf Feb. 3, 1942 2,116,559 Caruthers May 10, 1938 

